It is known to equip a host-vehicle with a trailer-detection system for detecting a presence and a dimension of a trailer towed by the host-vehicle. When the host-vehicle is towing the trailer, reflections of radar-signals from other objects not associated with the trailer (e.g. another vehicle in an adjoining lane) may interfere with the determination of the trailer dimension. The accurate determination of the trailer-size is essential for distinguishing the trailer from other targets for use in blind spot detection systems, lane departure warning systems, and other active-safety systems.